Variation of OLR with respect to the effective zenith angle used to compute the upward intensity (solid curve). The dashed curve shows the OLR computed by properly integrating the intensity over all zenith angles. The horizontal dash-dot curve shows the result of the classical hemispheric mean approximation (Toon et al. 1989), which systematically underestimates the flux by ~6 W.m⁻². The atmosphere model used has T_surf=500 K and P=1 bar.